The present invention relates to the deposition of thin films on a substrate by chemical vapor deposition and, in particular, to a method by which such films may be deposited uniformly in a pattern that has varying density.
Fabrication of electrical, optical or mechanical microdevices typically includes the sequential steps of substrate patterning (lithography), material subtraction (etching), and material addition (deposition). Often, it is desirable to deposit uniform thin films over dense three-dimensional microstructures. A collection of the microdevices, often referred to as microchips, may have different pattern densities. This requires the deposition of a thin uniform film over an area having a varying pattern density.
Because the area over which the film is deposited is larger for a more dense pattern, more chemical reactants are consumed in the vicinity of such dense areas. Increased consumption of such chemicals can lead to a localized depletion of chemically active species and a reduction in the growth rate of the film being deposited. The growth rate is the rate at which the film thickness builds up when the build-up increases linearly with time, or the total thickness of the film divided by the period of time required to develop the total thickness. When the growth rate is reduced in the more dense locations of the pattern, the film in the vicinity of the more dense locations of the pattern is thinner than at other locations where the pattern is less dense.
The variation of film thickness with pattern density will be referred to herein as the icroloading effect. The term xe2x80x9cmicroloading effectxe2x80x9d is commonly used in the art of reactive on etching (RIE), where it refers to the etch rate dependence on the pattern density. By analogy, we define the microloading effect for various deposition techniques. The deposition-based microloading effect should not be confused with the loading effect and the conformality (step-coverage) phenomenon. Both the loading effect and the step-coverage phenomenon are well known in the deposition art. The loading effect refers to the growth rate variation of a film from wafer to wafer. Accordingly, the loading effect has a much larger scale than the microloading effect. The loading effect is commonly observed in a Low Pressure Chemical Vapor Deposition (LPCVD) furnace where the composition of the reactive gas mixture changes along the furnace tube (reactor), leading to a growth rate variation along the tube. Due to the large scale of the loading effect the wafer-to-wafer growth rate variation can be easily reduced by either creating a small temperature gradient along the tube or introducing reactive chemicals through a plurality of apertures along the furnace tube. In contrast, due to the small scale of the microloading effect it is impractical to use the above-described techniques; it would be extremely costly to create a temperature gradient within a single chip or to provide means for independent delivery of chemicals to different parts of the same chip.
On the other hand, the step-coverage phenomenon has a much smaller scale than that of the microloading effect. In the deposition art, the step-coverage phenomenon refers to the film thickness (growth rate) variation with a single microstructure such as step, trench, or pillar. At the scale of less than several micrometers, step coverage is governed by very different physical mechanisms compared to the microloading effect. For example, the surface mobility of adsorbed chemicals can greatly contribute to the uniformity of the deposited films over 3-D microstructures. On the contrary, the surface mobility of adsorbed chemicals is not important at the scale of the microloading effect (several hundreds of micrometers). Therefore, the deposition-based microloading effect is easily differentiated from both the loading effect and the step-coverage phenomenon on the basis of substantially different scale.
As noted above, the microloading effect is ordinarily due to depletion of one of the chemicals participating in the reaction. The depleted chemical is referred to herein as a rate-limiting reactant. Even though the geometrical scales of microloading and loading effects are different, both effects are due to depletion of one of the chemicals participating in the reaction.
In the practice of chemical vapor deposition (CVD) of thin films, it is known that the loading effect is substantially reduced when the film growth is limited by the speed of the surface reaction and not by the speed of mass transport of chemicals. In order to perform CVD in such a surface-reaction-limited regime, it has been suggested that the growth rate be reduced by reducing the process temperature and pressure in the reactor. Most LPCVD furnaces are designed to operate in the surface-reaction-limited regime and have a relatively small loading effect. Due to the same reason, LPCVD furnaces have a relatively small microloading effect.
Recent trends in microfabrication clearly are directed toward shorter processing times, reduced thermal budget, and processing of large substrates. A new type of CVD reactor, characterized as a rapid thermal reactor, appears to satisfy these demands. A rapid thermal CVD reactor is a single-wafer unit that processes one wafer at a time. A schematic illustration of a first type of conventional single-wafer CVD reactor is shown in FIG. 1. This type of reactor is used to deposit various films of desired thickness onto a substrate 12. The chemical vapor deposition reactor includes a substrate susceptor 13 which has a resistive heater powered by a source 24. The heater can have multiple heating zones controlled independently by control circuitry (not shown). The mixture of chemicals includes one or more growth rate-limiting reactants and one or more other reactants, all of which can be optionally carried by a neutral gas. These gases are introduced into a showerhead 10 through a plurality of gas inlets 16, 18, 20. (In FIG. 1, as in other FIGS., the connections between the inlets and the showerhead are omitted for the sake of clarity.) After mixing in the showerhead the gases are injected into the process chamber through a plurality of apertures. The relatively small conductance of the apertures provides a substantial pressure difference between the process chamber and the interior of the showerhead. The gas conductance of the apertures can be varied center to edge to create a uniform gas flow in the gap between the substrate and the showerhead.
Reactants are selected according to the composition of the film being deposited on the substrate and the desired chemical reaction. For example, for silicon nitride (Si3N4) deposition, silane (SiH4) and ammonia (NH3) may be selected as the reactants. Generally, this choice is determined by the desired composition of the film being deposited on the substrate and an acceptable film growth rate.
The carrier gas is generally used to aid in the delivery of the reactants to the process chamber. The carrier gas is normally nitrogen (N2) or hydrogen (H2). In general, the carrier gas may be any of the inert gases, such as helium, neon, argon, or xenon. Gases other than nitrogen are more expensive and therefore are less commonly used.
After a film 14 of desired thickness has been deposited on substrate 12, the reactor chamber is evacuated by pumps (not shown) that are connected to outlets 22, 23. The back side of susceptor 13 is optionally purged during the film deposition by the introduction of a purging gas, such as nitrogen, which is introduced through an inlet 26. The purge reduces the deposition of the film onto the susceptor.
FIG. 2 is a schematic illustration of a second type of conventional CVD reactor that is arranged differently from the CVD reactor of FIG. 1. In the arrangement of FIG. 2, the growth-rate-limiting reactants, other reactants and carrier gas are introduced through inlet 26 and exit through outlet 28 which is connected to a pump (not shown). The reactor chamber is heated by lamp heaters 30 and 32 disposed above and below substrate 12, on which the film 14 is deposited. The reactor can be equipped with a wafer/susceptor spinning mechanism (not shown). In a typical single wafer CVD chamber of the first type (FIG. 1), a patterned wafer is placed 0.5 to 10 cm away from the gas source. In a typical single wafer CVD chamber of the second type (FIG. 2), a patterned wafer is placed 1.0 to 20 cm away from the chamber wall. The flow of gases is controlled with mass flow controllers (MFCs) 25, shown schematically in FIGS. 1 and 2. An MFC supplies a predetermined number of gas molecules per unit of time into the chamber. The chamber pressure is maintained constant with an adjustable valve 27 between the pump and the chamber.
Compared to CVD furnaces, a rapid thermal CVD reactor permits shorter processing time, greater manufacturing flexibility, and reduced thermal budget. In order to achieve these benefits, a rapid thermal CVD reactor has a high film deposition growth rate in comparison to the growth rate in a low pressure CVD furnace. The high growth rate of the deposited film in the rapid thermal CVD reactor makes this type of reactor very susceptible to the microloading effect. Furthermore, a rapid thermal CVD reactor may have a large temperature gradient that normally does not occur in a thermally stabilized low pressure CVD furnace. Such a temperature gradient may aggravate the microloading effect.
Accordingly, there is a need for a method for controlling the deposition of a film in a CVD reactor, particularly in a single-wafer, rapid thermal reactor, so that the microloading effect is reduced or avoided.
The present invention addresses the above-described need by providing a method for reducing the microloading effect in a CVD process for depositing a film on a substrate. This method is particularly useful in a single-wafer CVD reactor.
In accordance with a first aspect of the the present invention, the microloading effect is reduced by identifying a growth-rate-limiting reactant, calculating a dilution factor (the ratio of the gas flow rate of the growth-rate-limiting reactant to the total gas flow rate in the reactor); and adjusting the film growth rate and/or the dilution factor to satisfy a numerical criterion for reducing the microloading effect. The criterion is satisfied when the film growth rate is reduced, or the dilution factor is increased, so that the dilution factor is equal to or greater than a quantity which includes the film growth rate and the variation of substrate surface area as factors. The process generally has a minimum film growth rate (due to process throughput requirements) and a maximum dilution factor (due to film quality requirements). The film growth rate and dilution factor may be adjusted independently.
The substrate on which the film is deposited generally has a microstructure with a characteristic size, an aspect ratio and a pattern density. A second quantity may be evaluated based on these factors to determine the susceptibility of the substrate to the microloading effect.
In accordance with another aspect of the invention, the gap between the showerhead and the substrate in the CVD reactor is adjusted to satisfy the numerical criterion. In particular, the gap may advantageously be reduced to less than 5 mm, preferably to about 100 xcexcm. Providing a gap in the range 50 xcexcm-5 mm reduces a characteristic distance which is a factor in the above-mentioned quantity, so that the criterion becomes easier to meet.
In accordance with another aspect of the invention, the molecular weight of the carrier gas in the gas mixture is selected so that the mean molecular weight of the gas mixture is greater than the molecular weight of the growth-rate-limiting reactant. This choice of molecular weights eases the transport of rate-limiting chemical due to thermal diffusion. Since the above-mentioned quantity has as a factor an expression including these molecular weights, such a selection makes the criterion easier to meet in the presence of strong thermal gradients.
It is noteworthy that the present invention may be practiced in a high-throughput single-wafer CVD reactor and with a variety of film deposition processes.